The number of variant specific proteins (VSPs) in one Giardia clone was estimated to be between 130 and 150. Four copies of each gene appear to present and therefore between 30-40 different VSPs exist in this clone. Electron microscopy studies showed some Giardia lack a surface coat where VSPs are localized. Giardia infected athymic mice undergo antigenic variation but SCID mice do not suggesting non-cellular processes are responsible for loss of the initial VSP. HIV can enter and survive in Giardia; however, it is not clear if viable HIV are produced.